1. Field of the Invention
The present invention relates to a recording apparatus for performing an image recording utilizing an electrophotographic system for use in a laser printer, a copying machine, etc.
2. Description of the Related Art
A recording apparatus for performing an image recording utilizing an electrophotographic system has an electrostatic latent image carrier comprised of, for example, a photosensitive drum. The electrostatic latent image carrier has a photosensitive surface on its outer periphery. A charger device, an exposure device, a developing unit and a transfer charger device are arranged around the outer periphery of the electostatic latent image carrier.
As the electrostatic latent image carrier is rotated, the photosensitive surface of the drum is first charged by the charger device. Then the outer surface of the drum is exposed by the exposure device to form an electrostatic latent image on the outer surface of the drum. The electrostatic latent image on the outer surface of the drum is deposited, by the development unit, with a toner to provide a visual toner image. The toner image thus deposited is transferred by a transfer charger device to a transfer sheet.
After transfer has been made, a remaining toner (hereinafter referred to merely as a residual toner) on the photosensitive surface of the electrostatic latent image carrier is normally cleaned by a cleaning device, setting the recording apparatus in a ready state for next cycle.
Published Unexamined Japanese Patent Application Nos. 59-133573 and 59-157661 disclose a cleanerless recording apparatus, that is, an apparatus which, instead of cleaning a residual toner on the outer surface of a photosensitive drum, can collect the residual toner into a development unit simultaneously with the development of a latent image by the development unit and obtain substantially the same effect as that achievable by a cleaning step. Such a recording apparatus obviates the necessity of providing a cleaning device and hence can advantageously be made compact.
The aforementioned Japanese Patent Applications disclose a basic concept for a cleanerless recording apparatus, a summary of which is as follows:
The electrophotographic printer typically represented by a laser printer often employs a known reversal development. An ordinary recording apparatus using the reversal development method uses toner particles 2 charged with the same polarity as that on the charged surface of a photosensitive drum 1, the major arrangement being shown in FIG. 7. The electrostatic latent image is formed as a visual image such that the toner particles 2 are deposited on that non-charged area (or a less charged area) on the photosensitive surface of the drum 1, not on a fully charged area on the photosensitive surface of the drum.
In order to achieve such toner deposition, it is necessary that a voltage Vb (.vertline.V1.vertline.&lt;.vertline.Vb.vertline.&lt;.vertline.V0.vertline.) between a potential V0 on the charged area and a potential V1 on the non-charged area on the photosensitive surface of the photosensitive drum 1 be applied to a toner carrier 4 in the development unit 3.
The toner particles 2 deposited on the photosensitive surface of the photosensitive drum 1 is transferred to a transfer sheet 6 by a well known transfer charger device 5. At the transfer step, all the toner is normally not transferred from the photosensitive surface of the photosensitive drum 1, that is, some toner is left as a residual toner on the surface of the photosensitive drum 1 after the transfer step has been made. The residual toner 2' is collected by a cleaning device 7 and then charges on the photosensitive surface of the photosensitive drum are eliminated by a discharge lamp 8, followed by an electrostatic latent image forming step (a uniformly charging step by a charger device and an exposing step by an exposure device using a light beam 10).
In the cleanerless recording device, after a transfer step any residual toner 2' is left as it is, without using any cleaning device 7, until the developing step is reached. Simultaneously with the developing step, the residual toner 2' is collected into the development unit 3. Of the latent image formed through exposure by the light beam 10, a residual toner 2' after the transfer step, that is, a residual toner 2' present at a charged area (an unexposed area or a non-imaged area) is positively charged, by a charger device 9, with the same polarity as that on the electrostatic latent image. For this reason, the residual toner 2' is transferred to the toner carrier 4 by an electric field corresponding to a potential difference between V0 and Vb, that is, an electric field for suppressing the transfer of the toner particles 2 from the toner carrier 4 to the photosensitive drum 1. At the same time, the residual toner after the transfer step, that is, a residual toner on a non-charged area (an exposed area or an imaged area) receives a force acting from the toner carrier 4 toward the photosensitive drum 1 and stays deposited on the photosensitive surface of the photosensitive drum 1. A fresh toner 2 is transferred from the toner carrier 4 to the non-charged area on the surface of the photosensitive drum. In this way, cleaning is carried out simultaneously with the developing step.
Such a cleanerless recording apparatus eliminates the need for providing the cleaning device 7 and a spent toner box for storing a cleaned or a spent toner. It is thus easy to manufacture a simple and compact recording apparatus. Further, since the residual toner 2' following the transfer step is collected for reuse, the toner can be used efficiently and economically.
In the cleanerless recording apparatus, however, a ghost image sometimes emerges for the reason as will be set out below.
First, under a high humidity situation, the transfer sheet 6 normally absorbs moisture and becomes low-ohmic. As a result, there is a tendency that a larger amount of toner will be left on the photosensitive surface of the photosensitive drum 1. Any excessive amount of toner after the transfer step ensures no subsequent adequate cleaning. As a result, the toner 2' after the transfer step stays deposited on the non-imaged area and a positive ghost emerges against a white background of a transferred image. The ghost is called a "positive ghost" or a "positive memory".
Second, when any excessive residual amount of toner is involved after a transfer step has been carried out, an inadequate decline in the surface potential of the photosensitive drum 1 occurs due to the shielding of the light beam 10 by the residual toner, that is, the surface potential of the photosensitive drum 1 becomes a level (V1') intermediate between the potential V0 and the potential V1. In such a potential area, the development voltage becomes a level Vb-V1', a value smaller than a development voltage level Vb-V1 on the surrounding exposed area. Since, therefore, less amount of toner is transferred from the toner carrier 4 to the photosensitive drum 1, a white image emerges on an imaged area of a transferred image, the white image corresponding to a residual toner after the transfer step and being called a "negative ghost" or a "negative memory". This phenomenon prominently emerges in the case of a halftone image constituted by a set of dots and lines.
Published Unexamined Japanese Patent Application No. 62-203183 discloses a cleanerless recording apparatus having an arrangement as shown in FIG. 8. The apparatus is equipped with a conductive brush 12. The conductive brush 12, upon the application of a DC voltage by a DC power source 13, attracts a toner remaining after a transfer step from a deposited area under a coulomb force involved. This largely decreases an amount of toner remaining after the transfer step, thus preventing emergence of a ghost.
Since, however, the conductive brush 12 is set in contact with the toner remaining after the transfer step, it is difficult to uniformly charge the remaining toner, thus leaving a residual toner in an inadequately charged state after the transfer step has been carried out. This situation has been prominently encountered when more such residual toner is involved in particular. If this is the case, then a defect, such as a "ghost" or "memory", is liable to occur.